The present invention relates to a cryogenic superconducting device designed to operate at cryogenic temperatures and, more particularly, to a superconducting device for controlling the number of superconducting or normal conducting electrons, which tunnel through a semiconductor, by a voltage applied to a control electrode.
In the conventional switching devices operative at cryogenic temperatures, there is known a Josephson device. It has some advantages including a high switching speed and a small power consumption of several microwatts or so. However, this device is basically a two-terminal device, which is therefore not adapted for use in forming a circuit by the prior techniques accumulated in connection with the known semiconductor transistors, and employment of an AC power supply is requisite.
In an attempt to solve the problems mentioned above, a superconducting device has been developed by means of combining a superconductor and a semiconductor, as disclosed in Japanese Patent Laid-open No. 57-106186. In this device, a pair of superconducting layers are deposited on a semiconductor substrate with the respective end faces opposed to each other, and a control conductor is disposed in such opposed portions through an insulator layer. In such prior art, a superconducting layer formed on the semiconductor substrate is separated into two superconducting electrodes by photolithography and etching. Subsequently the portion thus separated is thermally oxidized to form an insulator film, and then a desired control electrode is formed by evaporation. For enabling the two superconductors of such device to interact in superconductive weak coupling through a semiconductor at cryogenic temperatures, it is necessary to dispose the two superconducting electrodes so close to each other that the distance therebetween becomes about 10 times the coherent length of an electron pair in the superconductor, i.e. less than 0.5 microns or so. However, in the conventional structure, it is impossible to facilitate formation of a control electrode in a narrow gap of 0.5 microns or less, whereby the dimensional precision cannot be enhanced in manufacture to eventually bring about difficulties in attaining high reliability and uniformity.
Another known superconducting device composed of a semiconductor with an electrode to control the characteristics thereof is JOFET (Hybrid josephson field effect transistors) proposed by T. D. Clark as reported in J. Appl. Phys., vol. 51, pp. 2736-2745, 1980. In such JOFET, an electrode of a superconductor is formed on a semiconductor substrate doped to a high concentration, or a dope layer is formed on a high-purity buffer layer and an electrode of a superconductor is formed thereon. In such device, control of its characteristics is executed by applying a voltage to the control electrode and extending the inversion layer from the control electrode side toward the semiconductor side. However, since the impurity concentration is high in the semiconductor layer immediately below the control electrode, the voltage to be applied to the control electrode reaches as high as several hundred millivolts while an output voltage obtainable is approximately equal to or even lower than that. Accordingly, it has been impossible heretofore to use circuits similar to those employed in the prior art of semiconductors.
In addition to the above, there are the following references related to superconducting devices.
1) IEEE Trans. Magn., vol. MAG-15, pp. 435-438. 1979 PA1 2) IEEE Trans. Electron Devices, vol. ED-28, pp. 1394-1397,1981 PA1 3) Japanese Patent Laid-open No. 57-176781 PA1 1) A pair of superconducting electrodes are formed on the two sides of a semiconductor, and a control electrode is disposed on the semiconductor through an insulator film so as to control a superconductive weak coupling state in the semiconductor between the superconducting electrodes. PA1 2) At least two superconducting electrodes are disposed adjacently to each other on a semiconductor layer (or substrate), and at least one control electrode is formed for controlling current flow between the superconducting electrodes to change the interelectrode superconductive weak coupling. And the distribution of impurities contained in the semiconductor layer is so arranged as to include at least one region of a high impurity concentration above the average and at least one region of a low impurity concentration below the average. Due to such varied concentration distribution of impurities contained in the semiconductor layer that serves as the superconducting electrode, the gain is increasable in controlling the interelectrode coupling state by an applied voltage.